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Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to
 idle enter/exit APIs
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>, Avi Kivity <avi@redhat.com>,
        Ingo Molnar <mingo@elte.hu>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>,
        Paul Menage <paul@paulmenage.org>
Date: Tue, 30 Aug 2011 16:13:57 +0200
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On Tue, 2011-08-30 at 16:11 +0200, Frederic Weisbecker wrote:
> On Tue, Aug 30, 2011 at 01:17:42PM +0200, Peter Zijlstra wrote:
> > On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > > But rcu read side critical sections (preemption disabled, rcu_read_lock(),
> > > softirq disabled) don't need the tick to enforce the critical section
> > > itself. 
> > 
> > Note that with PREEMPT_RCU only the rcu_read_lock() is actually an rcu
> > read side critical section, non of the others should be used as such.
> > Relying on preempt_disable(), local_bh_disable() and similar is broken
> > as per a long while ago.
> 
> Sure yeah.
> 
> My point was that the patchset doesn't care about all that anyway. Read
> side critical section still work as usual. What changes is the way we
> notice periods where we are *not* in rcu read side critical sections.
> This was previously partly made through the tick. Now it's still the case
> but we need to remotely wake up that tick first.

But you wake up the tick for any callback - systemwide - right? That's a
massive fail.